Your first cousin is your son's first cousin, once removed. Since that cousin's sister has the same parents, her relationship to your son is the same - first cousin, once removed.
Our cousins are our mothers sisters son :)
Your daughter's son's son is your great grandson. Your daughter and your sister's son are first cousins. Your great grandson and your sister's son are first cousins, twice removed. Your great grandson and your sister's great grandchild are third cousins.
Your sister's son is your nephew. Your son's daughter is your granddaughter. Your son and your sister's son are first cousins to each other. Your son's daughter and your sister's son are first cousins once removed to each other. Your grandchild and your sister's grandshild are second cousins to each other.
Your sister's daughter and your son are first cousins to each other. Your sister's daughter and your son's daughter are first cousins, once removed to each other. Your son's daughter and your sister's granddaughter are second cousins to each other.
Sam and his mother's sister's son are first cousins, just as you and Sam are first cousins. But you are not related to Sam's mother's sister's son.
If your sister-in-law is the sister of your spouse, her son is not related to your sister's son. If your sister-in-law is the wife of your brother, her son and your sister's son are first cousins.
Your sister's son and your daughter are first cousins to each other.
Your daughter and your sister's son are first cousins to each other. Your sister's son's daughter (her granddaughter) and your daughter are first cousins, once removed.
Your sister's daughter and your son are first cousins to each other.
Your daughter and your sister's son are first cousins to each other.
Grand-Nephew
Your son and your mother's sister are in a "Great Nephew/Great Aunt" relationship.